The present invention relates to devices for measuring stress on or strain of a material or structure, and more particularly to devices for measuring thrust such as that applied to shafts or bearings.
There is a need to make thrust measurements on propulsion shafts and machinery shafting such as on naval vessels. Accurate thrust measurements are needed on engine shafting such as ship propulsion shafting, especially on high speed gas turbine powered surface ships to determine propeller capability and the amount of drag on the ship due to hull fouling. Thrust measurements on rotating shafting experiencing torque loads are difficult to make. Conventionally, thrust measurements can be made utilizing one or more load cells placed in line with the shafting at a thrust bearing, which is usually difficult and requires the disassembly of machinery and costly design changes to incorporate the load cells. Alternatively, strain gages can be directly applied to the shafting, and the signals therefrom may be received from the rotating shaft via slip rings or radio telemetry. The shaft thrust is then calculated from these strain readings, knowing shaft material and geometry. However, shaft strains due to thrust, particularly those on ship propulsion shafting, are very low compared to strains due to shaft torque. Accordingly, only slight angular misplacement or misalignment of a strain gage on the shaft can cause substantial measurement errors in the thrust related strain output, due to shaft torque. Thus, such strain gages so mounted must be precisely oriented, and must be applied to the shaft by one who is expert with the application of strain gages. Temperature variations and centrifugal loading effects can also introduce errors in such thrust measurement.